Magnetic Random Access Memory ("MRAM") arrays of the type disclosed in the two above-incorporated U.S. Patents, and depicted in FIGS. 1a-b herein, include an array of magnetic memory cells (e.g., cell 9) positioned at the intersections of wordlines 1, 2, 3 and bitlines 4, 5, 6. Each cell includes a magnetically changeable or "free" region 24, and a proximate magnetically reference region 20, arranged into a magnetic tunnel junction ("MTJ") device 8. (The term reference region is used broadly herein to denote any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole.) The principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis ("EA") of the free region, and the ability to thereafter read this relative orientation difference.
More particularly, an MRAM cell is written by reversing the free region magnetization using applied bi-directional electrical and resultant magnetic stimuli via its respective bitline and wordline, and is later read by measuring the resultant tunneling resistance between the bitline and wordline, which assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or -EA), and if the reference region is a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (-EA/+EA).
An ideal hysteresis loop characterizing the tunnel junction resistance with respect to the applied EA field is shown in FIG. 2. The resistance of the tunnel junction can assume one of two distinct values with no applied stimulus in region 50, i.e., there is a lack of sensitivity of resistance to applied field below the easy axis flipping field strength +/-H.sub.c in region 50. If the applied easy axis field exceeds +/-H.sub.c, then the cell is coerced into its respective high resistance (anti-aligned magnetization of the free region with respect to the reference region) or low resistance (aligned magnetization of the free region with respect to the reference region) state.
Even if the magnetization pattern of the two regions forming the tunnel junction is simple, reversing the direction of magnetization in the free region during writing can actually affect one or both regions in unexpected ways. For example, the reversal of the free region during writing can result in the inclusion of a magnetic vortex or complex magnetic domain walls, pinned by a defect or by edge roughness. Because the junction resistance depends on the dot product m.sub.free m.sub.reference averaged over the junction area, inclusion of such complex micromagnetic structures in the magnetization pattern can substantially corrupt the measured tunnel junction resistance during reading.
For example, shown in FIG. 3 is the magnetization pattern in a free magnetic region 59 formed symmetrically about its easy axis EA in which a complicated wall structure is clearly evident between otherwise acceptable magnetization pattern regions. This overall magnetization pattern was attained from a nominally uniformly magnetized sample (both top and bottom layers originally pointing to the right), for which the easy axis bias was swept from +700 Oe to -700 Oe and back to +700 Oe. The reversal of magnetization evolved to a complicated structure as the field was swept from +700 Oe down to about -280 Oe. (The magnetization may not simply reverse at one critical field, but may evolve in a partially continuous and partly step-wise fashion as the net magnetization changes between the two possible opposing directions of magnetization.)
FIG. 4 is a hysteresis loop depicting the net direction of magnetization averaged over the device versus applied easy axis field for this corrupt sample. The non-square nature of region 150, resulting in a cell which will not predictably assume either one of its two states upon the removal of the easy axis applied field, is due to the evolution of such complex micromagnetic structures in the cell.
Some improvements in this situation are possible. For example, in the above-incorporated U.S. Patent Application entitled "INTENTIONAL ASYMMETRY IMPOSED DURING FABRICATION AND/OR ACCESS OF MAGNETIC MEMORY CELLS," the present inventors have disclosed a technique for avoiding the evolution of some of the undesirable micromagnetic structures in typical MRAM cells. Substantial improvements have been demonstrated, and in the best of cases no wall structures evolve during the cycling of fields used for reversing magnetization. However, for even these improved conditions, there can still be a substantial twist in the magnetization pattern.
This non-ideal behavior in the magnetization reversal process results in a reduction in the useful parametric window of operation at best, or a total collapse of the square hysteresis loop necessary for storage at worst. What is required, therefore, is an improvement in the techniques and structures for changing the free magnetic region between its magnetic states, which will minimize or eliminate the unwanted effects of complex micromagnetic structures which otherwise appear during this magnetization reversal process.